1. Field of the Invention
This invention relates generally to seismic cables, and in particular to seismic cables designed for laying on the sea floor. Such cables are installed in arrays on the sea floor and detect sound waves reflected from subterraneous strata in the earth's crust in response to energy pulses in the sea from an air gun.
2. Description of the Prior Art
Seabed laid seismic cables are known that include lengths of conductors that connect spaced sensor devices that are powered and interrogated from a surface vessel or fixed platform. Seabed laid seismic cables must be armored, because the cables are subjected to wear and abrasion when being installed on the sea floor and are required to be placed in mud and silt for long periods of time. Such cables also must be capable of withstanding marine growths, such as barnacles, etc.
Armored cables are generally known. For example, U.S. Pat. No. 4,439,633 discloses an armored cable and method of manufacturing the cable where an inner conductor is supported within a polyethylene tube, about which helically wrapped steel armoring is placed. U.S. Pat. No. 6,041,282 shows a number of elongated conductors (electrical and/or optical) which are arranged to be interconnected with a number of seismic sensor devices positioned at intervals along the cable. The '282 cable includes a central polyethylene tube. The sensors are placed at intervals along the tube, but power and signal conductors are arranged in stranded fashion over the tube. A polyethylene jacket covers the conductor, and steel wire armoring extends along the length of the entire cable. As the '282 cable has conductors and sensors characterized by an essentially constant diameter, armoring of the cable is not an unusual problem, because planetary armoring machines are known which helically wind steel strands about conductor lengths to produce an armored cable.
Seabed laid seismic cables desirably include geophone sensors in addition to hydrophones at instrumentation pods or sensor sections placed along the cable. A typical commercially available geophone may be approximately 1 inch in length and ⅞″ in diameter. The geophones take up more radial space than that of the conductor bundle, and the sensor assembly of which they are a part must be mounted in a protective housing in pods or sensor sections placed at intervals along the cable. U.S. Pat. No. 4,078,223 shows a seismic cable with sensor assemblies spaced along the cable of a larger diameter than that of the conductor sections between them. The '223 seismic cable has a plastic sheath placed over the conductor section and sensor assemblies, but no steel wire armoring. U.S. Pat. No. 4,241,427 shows Kevlar strands covering the conductor sections and sensor assemblies for protecting the cable. U.S. Pat. No. 3,939,464 shows a protective cage for sensor assemblies with a braided covering for the larger diameter sensor assemblies and the smaller diameter conductor sections.
U.S. Pat. No. 6,333,897 shows a housing for a sensor assembly interconnected with a central strength member of each of the conductor sections on opposite ends of the sensor assembly. The housing is made as small as possible so that, after the sensor assembly is interconnected with the conductor and the central strengths members, armoring of the conductor sections and of the housing can be provided. No disclosure is provided as to how steel wire armoring is applied to the in line housing and conductor housings, but illustrations in the '897 patent illustrate that the diameter of the housing is about less than twice the diameter of the conductor section.
U.S. Pat. No. 6,333,898 shows a seabed laid seismic cable where a sensor housing is placed outwardly of the conductor sections. At a location where sensor housings are to be placed, a sleeve is placed about the conductor section with an opening to connect jumpers between the sensor devices in the housing to conductors inside the sleeve. The conductor sections and sleeve are armored with stranded steel wire prior to attachment of the sensor housing to the sleeve. In other words, the sensor housing is not protected by the steel wire armoring.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a method and apparatus for applying stranded steel wire armoring to a seismic cable which includes conductor sections interconnected with sensor housings, where the steel wire armoring is applied in continuous strands about the exterior of the conductor sections and the exterior of the sensor housings.
An important object of the invention is to provide a method and apparatus for applying stranded steel wire armoring to interconnected conductor sections and sensor housings where the outer diameter of the sensor housing is about four times as great as that of the conductor sections.
Another object of this invention is to provide a steel wire armored seismic cable with sensor housings which are about four times as great in diameter than that of the outer diameter of the conductor sections.